At partial load of an engine, the exhaust gas quantity is reduced, which condition leads to a reduction in the supercharge pressure if the turbine inlet flow cross-section remains unaltered. Consequently, the engine does not receive enough air, the exhaust gas temperature rises and the danger of incomplete combustion increases. In order to permit driving without these problems, it is necessary to be able to adapt the turbine inlet flow cross-section during operation. This capability leads to a more or less constant supercharge pressure and smokeless operation over the whole control range.
Such a device for adjusting the flow cross-section is the subject of Swiss patent application No. 2609/82 of Apr. 29, 1982.
In that application, an adjustable annular slide makes stepless alteration to the turbine inlet flow cross-section possible. The adjustment of the annular slide occurs, by axial displacement. The annular slide is guided in a cylindrical bore in the gas casing, a radial clearance between it and the casing being essential. In order to prevent escape of the exhaust gas, the guide of the annular slide is provided with a labyrinth seal.
Guiding a relatively short cylindrical slide in a casing bore can cause difficulties with respect to jamming of the slide. In addition, friction losses and wear phenomena are unavoidable because of the high temperature of the parts rubbing together.